Summaries of newsworthy papers include Ozone destruction over an Atlantic holiday destination, Understanding four-legged fish, Cut off the food to spite the tumour, Volcanic explosions in the deep Arctic Ocean, It takes two and a feminizer for honeybees

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Scientists have identified what could be the largest impact structure in the Solar System, created on Mars at about the same time as the Moon-forming impact on Earth. The basin, now partially obscured by the Tharsis volcanic province, is elliptical in shape and over 10,000 kilometres long. Three papers in this week’s Nature provide fresh evidence for this huge, planetary-wide impact, and help to explain why one hemisphere of Mars is lower and has a thinner crust than the other.

The martian ‘crustal dichotomy’ has remained one of the most prominent, yet unexplained features on the planet for decades. A striking difference in topography between the smooth plains of the northern hemisphere and the rugged southern highlands indicates that for some reason the crust in the highlands is much thicker than in the north. There are two possible scenarios to explain the origin of this dichotomy: an impact of a large asteroid or comet; or large-scale mantle convection; but until now scientists have not had enough evidence to back up either theory. The problem is further complicated by the fact that 30% of the boundary is buried under more recent lava flows.

Jeffrey Andrews-Hanna and colleagues map the dichotomy boundary more accurately than before by using gravity observations to remove the contribution of the Tharsis volcanic bulge. They discover that the full extent of the boundary around the planet accurately fits an ellipse, which they name the ‘Borealis basin’. This basin — which they infer was formed by an oblique impact — is four times larger than any other known impact crater in the solar system.

In a second paper, Margarita Marinova and colleagues investigate the conditions under which a large impact event could have produced the observed crustal features on Mars without being destroyed completely. Using three-dimensional simulations, they test various impact angles, energies and velocities, finding that a low-angle impact produces melt that is largely contained and does not erase the evidence of the impact's occurrence.

Finally, Francis Nimmo and colleagues develop a series of high-resolution two-dimensional models to study the behaviour of the martian crust during an impact. They propose that a large impact, apart from excavating a crustal cavity of the correct size, would explain two additional observations: crustal disruption at the impact antipode, which they suggest is responsible for the observed antipodal decline in magnetic field strength; and the impact-generated melt forming the northern lowlands crust.

Increasing ozone in the troposphere over the past 150 years has been meddling with the climate and a full understanding of the factors controlling the ozone budget is required. The tropical marine boundary layer — the part of the atmosphere that interacts with the ocean — is the most important global region for loss of ozone, but observations in this region are extremely sparse. A paper in this week’s Nature presents new year-round measurements showing that ozone destruction is about 50 per cent more than predicted — and explains why.

Lucy Carpenter and her colleagues combined data collected at an observatory in the Cape Verde archipelago in the tropical North Atlantic Ocean with low-level aircraft observations. They found high concentrations of monoxides of bromine and iodine that ultimately originate from sea-salt aerosol particles and marine algae. These halogen derivatives are shown to be the mediators of ozone destruction, which turns out to be about 50 per cent more than predicted from atmospheric models that neglected to take any contribution from halogen chemistry into account.

The discovery of this omission is timely for future calculations of global ozone budgets and effects on climate.

With the skull shape of an early tetrapod, but proportions more closely resembling a fish, an exceptionally well-preserved fossil of the species Ventastega curonica is described in Nature this week. Seeming like an evolutionary half-way point between the lobe-finned fish Tiktaalik and primitive tetrapods such as Acanthostega and Ichthyostega, the discovery reveals that these animals diversified much earlier than previously thought, and helps reconstruct the sequence of events that took place during this transition.

The evolution from fish to land vertebrates occurred during the Late Devonian period — about 360 to 380 million years ago — and required many changes in physiology. Over the last 20 years, scientists have begun to piece together how this transition might have occurred, but a full understanding is hindered by the fragmentary nature of the fossil record.

Per Ahlberg and colleagues now describe a skull, braincase, shoulder girdle and partial pelvis of Ventastega, discovered in Late Devonian deposits in Latvia. The fossil is complete enough to allow the whole skull and parts of the limb girdles to be reconstructed, providing a picture of what an animal from this crucial evolutionary period looked like. Ventastega falls into the morphological gap between Tiktaalik and Acanthostega, with a tetrapod-like lower jaw, but retaining more fish-like fangs among other features. The findings also point to changes in skull shape during the transition — the eyes and snout became larger, but the skull overall began to shrink. The authors stress that although Ventastega fits nicely into the gap between previous finds, other fossils have shown substantially different morphological features, suggesting that diversification probably occurred much earlier than expected.

***This paper will be published electronically on Nature's website on 25 June at 1800 London time / 1300 US Eastern time (which is also when the embargo lifts) as part of our AOP (ahead of print) programme. Although we have included it on this release to avoid multiple mailings it will not appear in print on 26 June, but at a later date. ***

Researchers have uncovered the important role of a molecule in directing the formation of blood supply to tumours. The protein receptor VEGFR-3 could represent a new drug target for cancer therapy.

Angiogenesis — the process of growing new blood vessels — involves a complex network of signalling molecules known as growth factors and their receptors. Cancer cells manipulate this process by inappropriately sending out growth factors to generate their own blood vessel network providing nutrients for growth.

The receptor VEGFR-2 is already known to be important for angiogenesis and is a major target for drug development. In Nature this week, Kari Alitalo and colleagues establish that the related protein VEGFR-3 has a previously unknown role in signalling blood vessel ‘sprouting’. Using antibodies to block both receptors, the team found an additive effect in inhibition of tumour growth by effectively starving the cancer cells.

The findings could help to improve the efficacy of existing anti-angiogenic therapies and may prove useful in the combat against drug resistance, say the authors.

The 1,800-kilometre Gakkel ridge stretching across the eastern Arctic Basin was the site of a large earthquake swarm nine years ago. The mysteries underlying this event are revealed in a paper in this week’s Nature, thanks to striking photographic images of the site, collected during an International Polar Year expedition.

The swarm was thought to be linked with a major volcanic event, but its remote location and ice cover precluded data collection. Robert Sohn and colleagues conducted their high-definition survey using a new camera and sampling system designed for use beneath pack ice. Their images of the eruption area showed that it is blanketed by an extensive set of pyroclastic deposits — the largest such deposits found so far on the sea floor. This came as a surprise, because the axial valley of the ridge is at about 4,000 metres depth — previously considered too deep for explosive eruptions.

The authors also produced a high-resolution bathymetric map of this valley of ‘zero-age’ terrain, and found that it is filled with small flat-topped volcanoes about 300 metres high and 2,000 metres across. The findings show that large-scale explosive activity can occur even along the deepest portions of the global mid-ocean ridge volcanic system.

A paper in this week’s Nature describes the chemical synthesis of (-)-cyanthiwigin F, a marine natural product. This particular molecule is produced by a sea sponge, and the cyanthiwigin family are intriguing compounds that belong to a larger class of biologically active molecules known as the cyathins. Cyathins have a remarkable range of medicinal properties — among other things, they have been shown to curtail microbial activity, inhibit cell division in potentially cancerous cells and help to stimulate nerve growth.

Synthesizing this molecule in the laboratory was a challenge because of its complex molecular architecture: it contains several structural motifs that can be difficult to form in a highly selective manner. Brian Stoltz and John Enquist used an approach known as ‘double catalytic enantioselective alkylation’ to generate a key intermediate, which was then converted to the natural product in a series of additional steps. The researchers’ reported synthesis is relatively simple, opening the way for a thorough investigation of the molecule’s promising biological activity as well as providing an opportunity to prepare other members of the cyanthiwigin family.

[9] And finally… A feminizer for honeybees (AOP)
DOI: 10.1038/nature07052

***This paper will be published electronically on Nature's website on 25 June at 1800 London time / 1300 US Eastern time (which is also when the embargo lifts) as part of our AOP (ahead of print) programme. Although we have included it on this release to avoid multiple mailings it will not appear in print on 26 June, but at a later date. ***

Human gender is controlled by the X and Y sex chromosomes, but honeybees rely on an altogether different system. Their sex is governed by the combination of variants within a single master regulator gene called csd. A paper in this week’s Nature has discovered a key female-determining gene — known aptly as feminizer (fem) — that is located upstream of csd and from which csd is derived.

Male honeybees develop from eggs that have two copies of the same variant of the csd gene, whereas females have different variants. Martin Beye and colleagues found that the fem product also comes in two versions, one of which is defunct — they were able to generate male bees simply by shutting down expression of the female-specific version using a technique known as RNA interference.

Treating the female csd variant in the same way delivered the defunct, male-specific version of fem. The findings indicate that fem acts as a switch in the honeybee’s sex-controlling pathway during development.

***These papers will be published electronically on Nature's website on 25 June at 1800 London time / 1300 US Eastern time (which is also when the embargo lifts) as part of our AOP (ahead of print) programme. Although we have included them on this release to avoid multiple mailings they will not appear in print on 26 June, but at a later date. ***

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